Complexes of DNA gyrase with defined DNA fragments, previously studied by electrodichroism, have been further investigated by neutron scattering and dynamic light scattering. The results are compatible with the previously proposed model of a single loop of DNA of 110 base pairs bound to the enzyme, with tails of DNA emerging which become folded back onto the protein core when ATP or one of its nonhydrolyzable analogs is added. Nucleotide processing by DNA gyrase has also been studied. We found that under certain conditions the supercoiling reaction displays a great deal of "slip", in that the limiting supercoiling is well below the level would fully utilize the free energy of hydrolysis of ATP. A kinetic analysis of this situation has been devised.